1. Field of the Invention
The present invention relates to techniques and apparatus for harnessing the kinetic energy of a moving fluid stream and more particularly to a cascade of airfoils or hydrofoils oscillating in a fluid stream to produce useful work.
2. Background of the Invention
The recent search for alternate sources of energy has caused a renewed interest in utilizing the inexhaustible kinetic energy of moving fluids such as the wind, streams and even the oceans. While the windmill is the simplest example of a device capable of harnessing such energy, it has limited utility because its size is restricted by the centrifugal forces produced with large rotating blades.
One alternative to the windmill is disclosed in U.S. Pat. No. 4,024,409 to Peter R. Payne. This patent discloses a device including a wire which is oscillated by the shedding of vortices therefrom which oscillations are then converted into useful work. Like the aeolian harp, the Tacoma Narrows Bridge and a street sign flapping in a strong gust of wind, this type of oscillatory movement is induced when vortices are shed from a blunt body at a frequency in resonance with the natural vibration frequency of the object. This patent also discloses the use of a single blade oscillated in response to wind conditions. However, like the windmill, the amount of energy which can be harnessed by such a system is quite limited. Furthermore, such oscillations are due to the Karman vortex street phenomenon rather than the aeroelastic phenomenon of wing flutter.
U.S. Pat. No. 3,995,972 discloses a device including a stack of rigidly interconnected airfoils positioned in the wind. By sequentially varying the angle of attack of the airfoils, uniform oscillatory motion is produced for reciprocating a rod which in turn drives an output device. The disadvantage of this system, like the windmill, is that there is no way to compensate for variations in wind velocity to assure a substantially constant output.
It has also long been known that a great amount of energy is available when an airfoil is subjected to the aeroelastic phenomenon of flutter. However, studies of this phenomenon have been directed solely to preventing its occurrence since, if left uncontrolled, instability and eventual destruction of the airfoil results.
Finally, some work has also been done on the production of negative drag in the case of a single oscillating airfoil. See Garrick, I. E., Propulsion of a Flapping and Oscillating Airfoil, NACA Report No. 567, May 1936.